Variable resistor element



April 16, 1968 R. L. JONES 3,378,805

VARIABLERESISTOR ELEMENT Filed June 30, 1965 I'I/ HEL/XEE GUIDE ADJUSTMEMT E'0LL3 DRAW ROLLS\@\ :"--1 L O I O /8/ 60A TIA/6 N Z6 APPL/CATOE HEL/CALLY FORMED ELEMENT ROLL 457EA l'f/TEA/EE TENS/ON IN COMPE55ION INVENTOR. Pass/27 L. Java's United States Patent 3,378,805 VARIABLE RESISTOR ELEMENT Robert L. Jones, Riverside, Califi, assignor to Bourns, Inc., a corporation of California Filed June 30, 1965, Ser. No. 468,508 3 Claims. (Cl. 338300) ABSTRACT OF THE DISCLOSURE A resistance element of helical configuration adapted for use in potentiometers and like variable resistors, the element comprising an elongate mass of resistive material adherent on the inner surface of a relatively rigid wire, the latter being stressed by being curved in such a way as to place and maintain the resistive material under longitudinal compressive stress whereby likelihood of transverse cracking under tensile stress, and resultant circuit-opening or degradation, is avoided.

The invention hereinafter disclosed pertains to variable resistors such as otentiometers, and more particularly to improvements in arcuate or helical resistance elements in and for such resistors.

Variable resistors comprising arcuate or helical wire wound resistance elements electrically connected between two terminals and arranged to be wiped or brushed by a movable contact or wiper that is connected to a third terminal are common and well known in the prior art. The resistance elements are produced by wrapping upon enamel-insulated wire-like cores or mandrels slightly spaced-apart turns or convolutions of resistance wire, and bending the wound structure into arcuate or helical form. In some instances the resistance wire is bare, in which case care is exercised that upon bending into arcuate configuration adjacent convolutions of the resistance wire are not brought into electrical contact. In other instances the resistance wire is insulated, whereby adjacent convolutions of the wire are prevented from coming into electrical contact with each other; and an arcuate or helical bare contact zone for electrical contact with the wiper or brush of the variable resistor is provided by abrasively removing the insulation from aligned portions of the several turns of the wire. The removal of insulation as indicated may be performed either before, or after, the element is bent into arcuate or helical form.

The several characteristics of the aforedescribed wire wound resistance elements are well known. For example, electrical resolution, that is, the minimum change of resistance that can be effected by movement of the movable wiper or contact, is limited to the resistance of one convolution of the resistance-element wire. Thus, the variation of resistance exhibited between either end of the resistance element and the wiper or slider contact is of a step-bystep character, the steps of resistance variation being no smaller than the resistance of one turn of the resistance wire. Another characteristic of such wire wound resistance elements is the susceptibility to movement of one or more of the convolutions of resistance Wire longitudinally of the core or mandrel either during winding-on of the wire during bending of the element into arcuate or helical form. When the resistance wire is thus unevenly or nonuniformly distributed along the core, variation of resistance incident to the movement of the wiper contact is even more irregular. One proposal for preventing nonuniform distribution of the resistance wire on the core or mandrel has been to form a helical groove in the mandrel and wind the resistance wire in the groove; the seating of the wire in the groove serving to prevent, to some extent, displacement of the wire. However, if nonuniformity of dispersal of turns along the core is to be avoided, exact correlation of translatory movement of the core and movement of the grooving tool is required, and that is substantially impossible of attainment, since drawmg of the core is accomplished by frictional drawing rolls. In addition to the previously noted characteristics that contribute to poor resolution, there may be noted the lack of uniformity of resistivity of the resistance wire, and nonuniformity of the physical size of the wire, both of which contribute to poorer resolution.-

As a consequence of the impossibility of attaining desired fineness of resolution in many instances, and of the impossibility of attaining very high values of total resistance (TR) with wire-wound elements of small physical dimensions, there were developed several vvarieties of resistance elements formed as films or blocks of composition or the like, the latter being composed of carbon or of carbon and other conductive material dispersed in a hardened binder. For example, as disclosed in U.S. Patent No. Re. 25,479 and art to which reference is therein made, a resistance element is made by molding a mixture of acetylene black (carbon) and synthetic resin. Also, for example, as disclosed in U.S. Patent No. 3,099,578, an element is made by coating an insulator surface with a mixture of carbon, finely divided silver and finely divided iron dispersed in a phenol-formaldehyde resin binder. So that the films or blocks of resistive composition thus produced could be sufficiently wear-resistant as to withstand repetitive brushing by the wiper or contact without undesirable degradation of electrical properties, the compositions were made to be hard and wear-resistant. As a result of the element being hard enough to be wear-resistant, it was subject to cracking and/or crazing incident to repeated expansion and contraction caused by alternate heating and cooling during use.

The present invention is aimed at eliminating all of the above-mentioned undesirable characteristics of the prior-art resistance elements and is aimed at providing a resistance element of arcuate or helical form the resistivity and total resistance of which can selectively be very high or very low and the hardness of which is such as to be extremely resistant to deterioration by brushing by the wiper contact, yet which, due to a novel structural characteristic, is entirely immune from cracking or other fracture incident to repetitive cycling to and from high temperatures. Also, the provided element is characterized by so-called infinite resolution and by extremely low variation of resistance with variation of ambient temperature.

Briefly the invention comprehends the utilization of a hardenable resistive composition in viscous liquid form for forming a longitudinally-extending strip-like layer or coating -on a selected sector or portion of an elongate wire base or similar mandrel while the latter is moving along a linear path, hardening of the composition at elevated temperature and in situ during movement of the mandrel along a linear path which preferably is a continuation of the previously noted path, and creating permanent compression stress in the resistive composition whereby during subsequent temperature-cycling in service the composition is effectively prevented from cracking. The compression stress is created by so bending the core wire or mandrel that the cured composition is reduced in length, as by deforming the core or mandrel into arcuate or helical shape with the composition on the inner surface where it is subjected to compressive stress during the bending. The composition used is devised to provide, selectively and alternatively, high, low or intermediate resistivity by appropriate variation of the relative amounts of nonconductive binder, highly conductive finely divided metal, and carbon comprised in the liquid. Further, by appropriate selection of the ratio of conductive metal to carbon, the variation of resistance with temperature is controllable concurrently with the control of resistivity. Compositions or mixtures of carbon, silver and iron, for example, have been devised with the noted ends in mind, as are disclosed in US. Patent No. 3,099,578 to Hunter. Other mixtures, providing other examples of the prior art in the field of resistive mixtures or compositions, are set out in the disclosures comprised in Patents Nos. 2,689,294; 2,440,691; 1,954,353; 2,281,843, and other patents therein cited.

Further, in a preferred procedure for producing a resistance element according to the invention, the elongate base (mandrel or core), preferably in the form of an insul ated wire of round cross-section, is drawn from a supply, straightened as necessary, passed through a coating device which applies a continuous layer of the viscous adhesive resistive liquid composition onto a portion (usually about 120) of the peripheral circumference of the core, thence is drawn by drawing means through an oven or the like along a linear path whereby the volatile component (e.-g., an alcohol in which the resistive materials are suspended) is driven off and the adhesive binder (e.g., a thermosetting resin such as a synthetic-resin varnish) is substantially cured or set, and the elongate structure is then immediately formed into a curved (arcuate or helical) configuration with the composition disposed along the interior face of the curved core. Conveniently the bending operation is performed by a wire-helixing device substantially identical to that employed in forming spring wire into coil springs, the helixing device or helixer being disposed for rotation about an axis aligned with the aforementioned linear path of the core, whereby the bending or helixing of the mandrel can be so adjusted that the applied coating of resistive material is disposed on the interior surface of the helix or coil as the latter is formed. It will be evident that as the core is bent into curved configuration the outer half (approximately) is subjected to tensile stress and the inner half (approximately) is subjected to compressive stress, and that actual tensile elongation and compressive shorten ing, respectively, occur. Thus the applied resistive composition, being on the inner surface of the curved core, is similarly subjected to compressive stress and is slightly compressed and maintained under residual compressive stress and strain. The residual compressive stress is maintained during subsequent heat-cycling of the element during normal use, since the temperatures reached are below the Curie points of any of the materials involved.

Subsequent to bending into arcuate or helical form, appropriate lengths of the curved structure comprising the insulated core and applied adherent composition are severed by appropriate shearing or cutting means. The latter may be automatic shearing devices such as are used in the coil-spring manufacturing art, or manually-operated means such as cutters of the pliers type commonly used for severing wires.

In the preceding general description of the invention it is made evident that it is a principal object of the invention to provide improvements in resistance elements for variable resistors.

Another and more specific object of the invention is to provide a resistance element comprising an elongate base and a continuous elongate resistive structure thereon under compressive stress sufficient to prevent fracture incident to repetitive heating and cooling cycles.

An additional object of the invention is to provide an arcuately curved resistance element comprising an insulated core device arcuately curved and a resistive composition adherent on the inner curved surface of the core device and there held under longitudinal compressive stress.

Other objects and advantages of the invention are hereinafter stated or made evident in the appended claims or the following detailed description of a preferred embodiment of the invention and typical means for producing the same, references therein being made to the accompanying drawings forming a part of this specification.

In the drawings:

FiGURE 1 is a diagrammatic representation to no particular scale of principal components of apparatus used in producing resistance elements according to the invention and illustrating the steps of the method of producing such elements, together with material from which the elements are formed;

FIGURE 2 is a pictorial representation to an arbitrary scale of an exemplary helical resistance element according to the invention; and

FIGURE 3 is a view depicting a greatly enlarged transverse section and a very short length or extent of the exemplary element depicted in FIGURE 2 and produced by apparatus shown in FIGURE 1.

Referring first to FIGURE 1, an exemplary core or mandrel 12 in the form of enamel-insulated copper wire is drawn from a source of supply thereof such as a rotatable reel or spool 14. The core or wire is pulled through a straightener 16 of the type commonly used in wireworks, which device extensively removes abrupt changes of configuration of the core or wire. From the straightener the core is drawn to an applicator device '18 via guide roll means (which may merely pass the wire along a continuing straight course but which for convenience of illustration is shown as changing the course of the core), the applicator device comprising adjustment means whereby the amount of resistive material applied to the core may be closely regulated. The applicator device comprises a freely rotatable roll 18r arranged to be rotated by gearing driven by the adjustment means or, as depicted, by frictional draft of the core or wire on the applicator roll. The applicator roll 18 is arranged to dip into a liquid dispersion of resistive material contained in a reservoir 18v, and to carry a supply of the viscous dispersion to the area or zone of contact between the roll and the core 12. As is evident, a layer of the somewhat adhesive dispersion will adhere to the passing core and form on the latter on elongate continuous strip-like layer or coating covering a certain part of the surface area of the core. The amount of the resistive dispersion that is applied to the core may be regulated by regulation of the depth of the dispersion in reservoir 18v, or by regulation of the viscosity of the dispersion through variation of the proportion of volatile medium therein, or both. Also, the regulating means, such as means comprising adjustable regulating rolls 18a between which the core passes, may be used to vary the extent to which the core is made to course around the periphery of the applicator roll 181-, to thereby regulate the amount of the resistive mix or dispersion applied to the core per unit length of the latter. The level of the dispersion or resistive mix in reservoir 18v (or the extent to which the rotating applicator roll 18;- dips into the mix) may be regulated in obvious ways, such as by changing the rate of addition of the mix to the reservoir, by changing the elevation of the reservoir, and the like.

From the applicator device the now-coated core or Wire is drawn under tension along a substantially straight path through an oven 20, which is preferably electricallypowered and closely regulatable whereby a desired temperature may be maintained. As the coated wire or core passes through the oven the volatile component or components of the resistive dispersion are driven off and the resinous binder cured, so that by the time the coaed core reaches the draw rolls 22 the latter can without harm to the coating exert upon the core the frictional draft necessary for pulling the core through the aforedescribed devices. The draw rolls 22 are driven at regulatable speed by motor-driven gearing (not shown and not per se of the present invention), whereby the speed of travel of the core past the applicator device and through oven 20 can be set and maintained at a constant determined value. The ambient temperature in oven 20 is regulated by adjustable thermostatic means forming a part of that commercially-available unit; thus as the thickness of the coating applied to the core is changed,

or the speed of draft of the coated core through the oven is changed, or the composition of the coating is made more viscous, the curing can be regulated to suit and such that by the time the core reaches the draw rolls the coating is capable of successfully resisting the mechanical operations of drawing and helixing.

From draw rolls 22 the coated core passes to means effective to uniformly bend the core into arcuate or helical form with the coating disposed on the inner radius of the .bend or coil. Such means, as depicted, is a helixer '24 having helixing rolls of conventional construction and arrangement but supported in a housing attached to a worm wheel 24w supported for rotation around the axis defined by the linear path of the core. The worm wheel 24w is engaged by a driving Worm gear 24g, rotation of which, manually or by automatic means, rotates the helixing structure proper around the noted axis. By the described rotary means the helixdorming rolls can be brought into and maintained in exactly the positions required to form a helical coil of the coated core, with the coating disposed equally on opposite sides of the innermost radius R of the curve of the coil, as indicated in FIGURE 3. The radius of curvature to which the arcuate or helical extents of the coated mandrel or core are bent is determined by the sizes and spacing of the he1ixing rolls in the helixer, as is well understood in the wire- Working and spring-making arts, and the rolls and spacings are selected as required by any desired radius of curvature. If at commencement of a length of coated core the bending is not such as to dispose the coating on the inside of the formed coil, gear 24g is rotated to rotate the helixer rolls to the required position or attitude in which the bend is formed with the coating on the inside radius.

In FIGURE 3 there is depicted, somewhat exaggerated, a section and short length of arcuate resistance element formed by the apparatus hereinbefore described. As there indicated, the core or mandrel of the resistance element comprises an inner metal conductor 120, preferably of copper, encircled by a continuous tenaciously-adherent jacket of insulation 12i, preferably of enamel. The completed element comprises, in addition, an applied closelyadherent coating 12r of resistive material disposed equa1- 1y on both sides of the inner radius of curvature R of the core (FIGURE 2), the resistive coating 12r and the innermost half of the core 12c being under compressive strain due to the stresses applied during bending of the core, as indicated in FIGURE 3. Due to the compressive stress applied to coating 121', and the continuing compressive strain therein, the resistive coating is remarkably resistant to cracking and/or crazing such as is experienced with hard wear-resistant coatings and films applied on flat surfaces when such coatings and films are subject to heat-cycling incident to normal use.

As indicated in FIGURE 2, the core or mandrel with the applied resistive coating is formed into arcuate or helical form by the helixer. From the extremely long or continuing coil 26 that is formed by the helixer, lengths or extents of the coil are readily severed to provide arcuate resistance elements of less than one turn, or helical elements of two, five, ten, or other numbers of turns, as may be desired. In normal practice the helixer is adjusted to form helical coils of diameter slightly greater than are required in multi-turn otentiometers, for example, whereby the coil may be reduced in diameter and allowed to expand into position in the potentiometer barrel. However, as is evident, other practices may be followed without adversely affecting the compressivelystressed resistive coating or resistance element proper, 121'.

The previous description of the invention makes evident the complete attainment of the aforestated objects. In the light of the disclosure changes and modifications will occur to others, and accordingly it is not desired to restrict the invention to the exact details of the exemplary structure other than is required by the appended claims.

I claim:

1. A resistance element for potentiometers and like variable resistors, said resistance element consisting essentially of an elongate insulated-wire core formed into a helical coil and a helical unitary mass of homogenous resistive material of crescent cross-section adherent to the inner surface of said insulated-wire core on both sides of radius of the helical coil, and said insulated wire core having the outer portion of the helix thereof under tensile stress whereby to hold the inner portion of the helix thereof and said elongate unitary mass of resistive material under longitudinal compressive stress whereby transverse cracking of the mass of resistive material is prevented.

2. A resistance element according to claim 1, said insulated-wire core being of circular cross-section.

3. An elongate helical resistance element especially adapted for use in multi-turn helical-element variable resistors, said resistance element comprising an elongate helical core having an insulative surface along at least the inner surface of the convolutions thereof, and said element further comprising an elongate integral homogeneous mass of resistive composition of crescent cross section disposed along said inner surface and firmly adherent thereto and held under longitudinal compression by said helical core whereby transverse cracking of said resistive mass is prevented.

References Cited UNITED STATES PATENTS 2,060,114 11/1936 Podolsky 338--308X 2,933,710 4/1960 Novack 338-308 FOREIGN PATENTS 122,417 4/1931 Austria.

DARRELL L. CLAY, Primary Examiner.

L. H. MYERS, Examiner.

E. GOLDBERG, Assistant Examiner. 

